Super Mistake Caused Super Voltage

My story is about aircraft equipment using 400Hz three-phase power. A piece of equipment was returned because it wasn’t working. It had burn marks in strange places, even though there was no voltage higher than the usual +/-15V on the backplane. It was an absolute mystery, suggestive of misuse.

After a lot of thinking about the circuitry, it finally occurred to me that a simple fault could apply a feedback path to the main switch-on relay, making it chatter. That is, as soon as it was switched on, a signal turned the relay off, and when power was lost, the relay went on again.

Relay chatter can be fast, on the order of 1 ms to 10 ms, and produces a fast risetime, or arcing (if there is inductance in the circuit), as the contacts make and break. Relay chatter generally results in high RF content, which is sometimes used as a source of random RF interference.

I recollected that a few months earlier, a substation power transformer went faulty, with the ground open circuit. That doesn't sound like a lot, but since the transformer supplied several buildings, with each of three phases supplying random loads in each building, the missing ground meant that the center-tap of the transformer could swing anywhere in -- and even out of -- the delta of three phases. So, equipment could be over-voltaged from normal 240V to way over 440V. A lot of PCs and test gear were destroyed!

What was happening with the aircraft equipment was related. When the power-on relay chattered, each of the contacts was making and breaking at slightly different times. The L-C input filter was resonating all over the place, generating enormous voltages, which were breaking through where they should never have been, causing arc scorches. This had never happened in earlier equipment, but in the model I was analyzing, some engineering upgrade had added inductors to the input filter.

The cure was to revise the logic a little, so that the original fault would not cause relay chatter.

—This entry was submitted by Rod Dalitz and edited by Rob Spiegel.

Rod Dalitz studied physics at university in England. After graduating, he worked in electronics: in precision weighing, brewery automation, and for the last 25 years, in avionics. He is now retired.

It's called surge impedance, and it's the exact same thing that burned loads & nearly caused fires in another Sherlock Ohmsepisode:Noise Messed With the Automation System.Here is my explanation, copied & pasted verbatim:

It's called surge impedance Zo, which is defined as √(L/C), where L is the inductance in Henrys, and C is the capacitance, which for a coil is the interwinding stray capacitance.

The back EMF V= Zo (δI/δT), and it will have an oscillation frequency 1/(2π√LC). When you open up contacts in an inductive load, δI/δT goes to ∞: You see this as an arc when you unplug an iron; and also when relay & motor starter contacts are switching off an inductive load.

This is also why contactors have serious current deratings when switching off DC: Once the arc is established and current flows through the ionized channel, there is no zero crossing to extinguish the arc, as occurs with AC,

When dealing with AC, you design using the peak (not RMS) value of the load current when calculating, because you don't know where in the AC cycle the contacts will open.

Note: Those of you who are RF jocks will quickly recognize Z(o) = √(L/C) as the equation for the characteristic impedance of a transmission line: Yes, it's the same thing.

I am looking for a low 1-5v chatter method. Does anybody have any ideals. I would like to see the sch/design/specs of circuit. Because I am working on a new oxidizing graphene method similar to this fault. Maybe this is what my mom meant, when she always use to say to me, "I could be a preacher!. See what we can learn from chatter? Please contact me irwin@911ec.com wh circuit design info.

I am frequently confronted with supposed "Upgrades" that solve some sort of immediate problem, but lead to other problems somewhere else. To me this happens most often in assemblies. One of our customers is heavily involved in hood hinges and locking mechanisms in heavy over-the-road haulers. It is not at all unusual to begin to alter tooling to produce a part which will clear something that has been added under the hood only to be stopped before completion because the addition has been removed.

Years ago the company at which I worked sold the same die block to the same customer (3) times. We were producing a part and had to alter the die to produce a new version which did not work (for reasons I do not know), and we went back to the original design. This happened on two more occasions before product launch when they replaced the part altogether. So after paying for the tool and subsequent alterations they ended up with an expensive boat anchor and I had several healthy paychecks from overtime worked. It was a very enlightening experience in the operation of huge corporations where not everyone was on the same page.

One of the things this should remind us is that all LC circuits are resonant. I don't care if the resonance is way out of your operating range, something unintended will stimulate it. Always make sure they are adequately damped.

If, when they added that inductor, they had also added the right resistor in the right place, the upgrade would have never caused a problem. Of course, if they had done that task right, the dropout problem in the relay might never have been noticed.

When I arrived at Sunbeam/Aircap, the Chief Engineer proudly showed me the ceramic brusholder they had designed to keep the bakelite holder from seizing on the carbon brush of their motor. By examining some failed parts, I saw the spring collapsed and discolored, plus the solder gone from where the copper braid wire was connected to its metal plate - at the REAR of the brusholder. They brought 120 VAC to the brusholder by a wire attached to a small metal plate that was held against the brush braid wire plate by its spring. Vibration was bad enough to cause chatter between the plates, with heating enough to melt the solder. Once the solder left, the spring tried to carry the current, but its steel turns quickly annealed and relieved the pressure that held the brush against the armature, causing the ring of fire that made the bakelite swell and sieze the brush. About $50 k was spent on tooling alone for that "upgrade". Instead, we added a quick-connect tab intgral to the rear plate and connected power directly, eliminating a part from the BOM, saving money and warranty costs. Mike Harris

I understand that a reboot can fix the problem. But I don't know if that really solves the root cause of the problem. Somewhere there's something in the code that caused this problem. And if rebooting means the system will work right now, it doesn't necessarily mean the problem is solved.

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